This invention relates to a process for the catalytic conversion of ethene to provide an oligomer product which is rich in C4's, especially, isobutylene.
The process employs a selectivated ZSM-35, which exhibits unique characteristics under the process conditions described below.
Ethene is a by-product of petroleum refining and is, as well, a commodity petrochemical. Today much of the ethene generated during refining and some of the lower concentration ethene streams from petrochemical production are burned as fuel. Significant economic value could be derived if these ethene-containing streams could be processed to generate higher boiling, more valuable hydrocarbons from the ethene. Interest in making C4 olefins from ethene has increased in recent years as a way of utilizing ethene and also as a way to make 2-butene for alkylation purposes. In addition, since isomerization within the C4 olefin group is well-known, linear C4 olefin can be used to make isobutylene, an intermediate for t-butyl methyl ether (MTBE).
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Such a technique has been developed by Garwood, et al, as disclosed in European Patent Application No. 83301391.5, published Sep. 29, 1983.
The prior art teaches conversion of C.sub.2 + monoalkenes to an equilibrium olefin mixture under conditions which maximize the formation of higher olefins. For example, zeolites such as ZSM-5 are known to convert lower olefins to higher olefins. Conversion of lower olefins, especially propene and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however low severity conditions do not convert a major fraction of ethene. While propene, butene-1, and others may be converted to the extent of 50% to 95% at temperatures up to 400.degree. C. and moderate pressures from ambient to 5500 kPa, only about 10% to 30% of the ethene component will be converted using HZSM-5 or similar acid zeolites, according to U.S. Pat. No. 4,717,782 to Garwood et al. The olefin interconversion process must cope with undesirable side reactions which yield aromatics and paraffins, the presence of which is acutely noticed at the relatively high temperatures (&gt;700.degree. F.) at which ethene conversion and formation of i-C.sub.4.sup.= and i-C.sub. 5 .sup.= formation is thermodynamically favored. In order to avoid such undesirable side reactions,-the '782 Garwood et al reference teaches the use of a bifunctional nickel-zeolite catalyst for oligomerizing ethene streams containing hydrogen and hydrogen sulfide at 100.degree. to 450.degree. C. and 200-3600 kPa (15-500 psig), wherein water is fed with the feedstock to prevent reduction of the nickel component.
Isobutylene is desirable inasmuch as it can be etherified with lower C.sub.1 -C.sub.5 aliphatic alcohols, to produce alkyl tertiary butyl ethers. The ethene conversion product comprising a high proportion of C.sub.4 olefins can be blended directly into "base" gasoline, or, to etherify all, or a portion of the product with lower C.sub.1 -C.sub.5 aliphatic alcohols. The latter option is especially advantageous because the etherification reaction proceeds apace and with gratifying selectivity.
Thus, it is of interest to find catalysts of increased activity which are able to convert ethene more selectively into butenes at higher feed rates, lower reaction pressures or with more dilute ethene streams and simultaneously minimize production of the higher oligomers. Moreover, given the increased demand for alkyl tert butyl ethers such as MTBE, it would be highly desirable to provide a process which produces not only linear butenes from olefins, but isobutylene as well.
It is known in the art that surface acidity of zeolitic catalysts can be modified by treatment with various reagents. U.S. Pat. No. 4,870,038 to Page et al discloses a process for producing substantially linear hydrocarbons by oligomerizing a lower olefin at elevated temperature and pressure with siliceous acidic ZSM-23 whose surface is rendered substantially inactive for acidic reactions, e.g., by contact with 2,4,6-collidine (2,4,6-trimethylpyridine, gamma-collidine). U.S. Pat. No. 5,015,361 to Anthes et al discloses a method for catalytic dewaxing which employs surface acidity deactivated zeolite catalysts. The reduction in surface acidity serves to reduce the amount of lower value cracked products obtained during dewaxing. U.S. Pat. No. 4,101,595 teaches the modification of zeolites by exchange and similar technology with large cations such as N.sup.+ and P.sub.+ and large branched compounds such as polyamines and the like. Bulky phenolic and silicating zeolite surface-modifying agents are described in U.S. Pat. Nos. 4,100,215 and 4,002,697, respectively. As disclosed in U.S. Pat. Nos. 4,520,221 and 4,568,786, zeolites which have been surface-deactivated by treatment with bulky dialkylamines are useful as catalysts for the oligomerization of lower olefins such as propylene to provide lubricating oil stocks.
Deposition of carbonaceous materials by coke formation can also shift the effective ratio of intra-crystalline acid sites to surface active sites, as disclosed in U.S. Pat. No. 4,547,613, wherein a zeolite catalyst is conditioned by contact with C.sub.2-16 olefin at 400.degree. to 1000.degree. F. at 0 to 100 psig for 1-70 hours. The conditioned catalyst provides an oligomerized olefin product having a high viscosity index. U.S. Pat. No. 5,234,875 to Han et al. teaches coke-selectivating zeolites under high pressure coking conditions for use in oligomerization of lower olefins. Other examples of coke-selectivation of zeolite catalysts are set out in U.S. Pat. Nos. 4,001,346 to Chu et al. and 4,128,592 to Kaeding et al. All of the foregoing references are incorporated herein by reference.